Power word: shield (new to healing)

So basical I created a priest about a week and a half ago. I am now level 70, and mainly leveled as a Shadow priest Questing with random here and there instances as a holy healz. In the past I had made a priest and leveled only in instances as a holy healz up to 70 (after cata), and had dont the same with a druid (before cata) up to 50-something. Doing that I realized I LOVED healing. My first toon in the game was a hunter, because my recruit a friend told me to pick that.

After I decided i wanted a priest healer I start a new one to learn the shadow tree better too, just cause I wanted to know how to dps well on the side too. I think I learn a tree better by leveling in it then trying to firgure it out after Im already max level. blah blah, but anyways my question would be about healing.

So when I go into a 5-man to I do all the buffs, and want not, get into the chakra state for Holy Word: Serenity, (I believe thats the right name of the one I'm talking about), then right before the tank pulls I do the power word: shield on him. and if the shield takes too much dmg and goes away before the 15 seconds are over ill use renew or flash heal or the serenity just because it cost like no mana. and I normally never run out of mana. The shield sometimes thats all I do the entire time and the tank never goes below 75% health. And I normally gain all the mana back after I do the shield before the 15 seconds are up.

For stats Ive been doin Int-spirit-(then Im not sure, not really much at low level)

So reading things about healing I dont see much of people using the shield. I want to be a raiding healer soon, and dont want to look like a dummy if Im using the shield and its not normally done. So I wanted to know peoples opinions about it. If I should be using it all the time? or when It comes to raiding its not worth using, or there are better things?

You're doing things mostly right since you're leveling, but your title is about PW:Shield so I'll talk on that. PW:Shield is a great spell that any priest can and should use to mitigate incoming damage. It is useful for a priest of any spec, however it becomes a much stronger spell at higher levels for some specs than others.

PW:Shield is really the strength of the Disc tree and it has several talents and specializations that increase the amount it absorbs. When you get into raiding, it becomes a big part of Disc healing, and for this reason Holy and Shadow priests in raids won't use it so often (since Weakened Soul sort of locks out Disc priests from using it on that target). For Holy, it doesn't have many of the talents that make it as strong as Disc but it can still be a useful spell. Talents like Body & Soul give it some utility with a mini-sprint, but it still won't absorb nearly as much as Holy as it would as Disc. At that point, you're using it for its meager damage absorption and its Body&Soul affect, both of which can be important and useful.

While leveling, it can be a useful spell and one that even Holy and Shadow can and should make use of, particularly in 5-mans where you get the opportunity to bring out your entire arsenal of spells. It's good to learn those applications now so you know where you can use them in the future. It won't be an often used spell by holy raiders, but it is one of the best when used situationally (either for the absorption or the speed boost).

Also, minor note. Mana may be fine now, but it becomes more difficult at 80+. A good point to note right now is to use Heal instead of Flash Heal when time and mana permits, as Flash Heal gets much more expensive and therefore much more prohibitive at later levels. Otherwise, play around with it just like you have been; sounds like you're doing just fine!

Also, from an efficiency standpoint (for Holy) Shield is too expensive, but it has great throughput. It's also pre-emptive meaning you can use it before damage hits. So it's useful sometimes even though it has bad efficiency.

For instance, in raid, when Magmatron acquires a target, or Monstrosity Gravity Crushes, I might shield if that person is already pretty low.

Also, any healing-reduction effects make Shield more viable. For instance, during the last phase of Cho'gall random players receive a debuff that nullifies all healing. PW:S is still effective. Same goes for Mortal Strike. If your tank gets hit with MS, shield is your most efficient response.

When someone is kiting, shield also provides Body and Soul (that speed buff). Very useful.